As is known, all electronic devices that include integrated circuits require at least one DC voltage supply and typically requires multiple DC voltage supplies. A DC voltage supply may be generated from an AC voltage source (e.g., 110 volts AC) or from another DC voltage supply (e.g., a battery). To generate a DC voltage supply from an AC voltage, the AC voltage is processed in a controlled manner. For example, a switch-mode power supply will rectify the AC voltage to produce a DC bridge voltage. Using one of a plurality of switch mode converter topologies (e.g., full bridge, half bridge, buck, or boost) an inductor is charged and discharged at a controlled rate to produce a regulated DC voltage supply.
To regulate the DC voltage supply at the desired voltage, a feedback loop is used. Typically, a resistance divider network is coupled to the DC voltage supply to produce a representation of the DC output that is provided to a controlled circuit. The control circuit includes an operational amplifier, a saw tooth generator, and a comparator. The operational amplifier receives the representation of the DC output and a reference voltage to produce, therefrom, an error signal. The comparator receives the error signal and a saw tooth signal, which is produced from the saw tooth generator, and produces, therefrom, a pulse width modulation signal. The pulse width modulation signal controls the charging and discharging of the inductor. Depending on the overall gain of the power supply, the DC output can be regulated within a few mVolts.
Thus, when only one DC output voltage is needed, a well-regulated power supply system as described above may be used. When multiple DC output voltages, or supplies, are needed, design choices must be made to optimize the performance of the multiple output power supply. If power consumption is not a significant issue, but well-regulated multiple DC output is, then linear regulators may be used with reference to a primary DC output supply. For example, assume that a five-volt and three-volt DC outputs are needed where the five volt is being produced by a switch mode power supply. To produce the three volts supply, a linear regulator is coupled to the five volts supply and regulated to three volts. While the linear regulator will accurately produce the three-volt output, it is inefficient since that for every three watts of output power produced, two watts are consumed.
In an alternate design choice, if power consumption is a critical factor, but regulation of auxiliary supplies, (e.g., the three volts in the preceding example) is not a critical factor, then a multi-tap transformer may be used in place of the inductor. A secondary tap on the transformer produces the auxiliary DC output and a primary tap produces the primary DC output. In this embodiment, only the primary output is regulated. Thus, as the load varies on the primary DC output, the auxiliary DC output will vary by as much as ten percent (10%).
In designs where both power consumption and well-regulated multiple outputs are significant factors, DC to DC converters are used. As is known, a DC to DC converter includes its own inductor and control circuit to regulate a DC output from a DC input. Thus, multiple inductors and multiple control circuits are needed. As with most electrical devices, size and cost are concerns. Thus, having multiple DC to DC converters to produce regulated power supply voltages is prohibitive to reducing size and reducing costs of such devices.
Therefore, a need exists for a method and apparatus of regulating DC output supplies without the above-referenced limitations.